The mitzvah as a doorway

והוא יושב פתח האהל שצריך האדם לידע אף כי יש להקב"ה נחת רוח ממעשה בו"ד וכל העולמות תלוין בהתעוררות מעש"ט מהתחתונים שעי"ז מתגלה השגחתו ית' עכ"ז הוא בפתח והיינו שהשי"ת עשה כן להיות נתלה דברים עליונים במעשה המצוה שע"י המעשה גשמיי נפתח המשכה במה שלמעלה ומשם יותר עד אליו ית' ממש.

"And he was sitting at the entrance of the tent" — a person must know that even though HaKadosh Baruch Hu has nachas ruach (satisfaction) from the deeds of flesh and blood, and all the worlds depend on the arousal of good deeds from those below, through which His hashgachah (providence) is revealed — nevertheless it is only "at the entrance." That is, Hashem arranged it this way, that exalted matters should hang upon the act of the mitzvah, so that through the physical deed an opening is made, drawing down what is above, and from there higher still, up to Him Himself, literally.

Avraham "sitting at the entrance" teaches a balance: our mitzvos genuinely give Hashem satisfaction, and the entire spiritual order depends on the good deeds of people in this world, which reveal His providence. Yet a person stands only at the "entrance" — the physical mitzvah is just the doorway through which influence is drawn down from the upper worlds, level after level, all the way up to Hashem.

אבל להאדם רק פתח כחודה של מחט.

But for the person, it is only an opening like the point of a needle.

Man's own contribution is tiny — no more than a pinprick of an opening. Echoing the Midrash, "Open for Me an opening like the eye of a needle," our small effort is all that is asked, and Hashem widens it from there.

אף שהוא כחום היום והוא בחי' אברהם אהבה שהוא ביטול באחדות גמור עכ"ז זכר כי הוא רק בו"ד בפתח האהל:

Even though it was "in the heat of the day" — which is the aspect of Avraham, ahavah (love), which is bittul in complete oneness — nevertheless he remembered that he is only flesh and blood, at the entrance of the tent.

"The heat of the day" alludes to Avraham's burning love for Hashem, a love that is total self-nullification into His oneness. Even at that exalted level of ahavah and bittul, Avraham never forgot his place — that he remains a human being standing humbly "at the entrance," not the source of the light but merely its doorway.

Summary: Avraham "sitting at the entrance of the tent" teaches the balance of avodah: our mitzvos truly give Hashem nachas and all the worlds depend on the good deeds of those below, yet a person stands only at the "entrance" — the physical mitzvah is a needle-point opening through which Hashem draws down His light. Even Avraham, whose burning love was total bittul into Hashem's oneness, never forgot that he is but flesh and blood at the doorway.